Theoretical Studies On Group VB Transition Metal Oxide Clusters: M₄O_x^-/0 And Nb_yTa_3-yO₃^-（M=Nb,Ta;x=8-10,y=0-3）

Early transition-metal oxides have many industrial applications in catalysis,materials, and surface sciences because of their diverse physicochemical properties. Inparticular, group VB transition-metal oxides are widely used as both catalysts andcatalytic supports for important chemical reaction processes, such as SO2â†'SO3oxidation, photocatalytic decomposition of water and selective oxidation ofhydrocarbons. With a size between those of atoms and macroscopical systems,gas-phase clusters may build a bridge to study the connection between macro-andmicro-systems. Several studies have suggested that a metal-oxide surface may beviewd as a collection of clusters of different sizes and isomers. Consequently,gas-phase clusters can be used as molecular models to mimic the geometric andelectronic properties of metal oxide surfaces/defects and provide fundamental insightsinto the mechanisms of catalytic reactions that will find use in the design of moreefficient catalysts. In this dissertation, we report the theoretical studies on a series ofgroup VB metal oxide clusters: M4Ox-/0(M=Nb, Ta; x=8-10) and NbyTa3-yO3-(y=0-3). A summary of our work is given as following:1. The electronic and structural properties of a series of tetra-niobium oxide clusters,Nb4On and Nb4On(n=810), are investigated using density functional theory (DFT)calculations. A trend of sequential oxidation is observed as a function of O contentuntil Nb4O10, reaching the highest oxidation state of Nb. DFT calculations areperformed to search for the lowest energy structures for both the anionic and neutralclusters. The global minimum of Nb4O9is formed by removing a terminal O atomfrom the stoichiometric Nb4O10cluster, creating a tricoordinated Nb site. Our MOanalysis shows that this Nb site has a pair of localized d electrons and it is essentiallya localized Nb3+site. Such defect sites are chemically active and may act as catalyticcenters in bulk oxides or catalysts.2. Density functional theory (DFT) calculations are carried out to investigate theelectronic and structural properties of a series of tetra-tantalum oxide clusters,Ta4On/0(n=8-10). Generalized Koopmans’ theorem is applied to predict the vertical detachment energies and simulates the photoelectron spectra (PES). The structuresand electronic states for the tetra-tantalum oxide clusters are compared with thecorresponding tetra-niobium oxide clusters. Molecular orbital analyses are performedto analyze the chemical bonding in the tetra-tantalum oxide clusters and to elucidatetheir electronic and structural evolution.3. In addition, we study the mixed metal oxide clusters: NbTa2O3-and Nb2TaO3-.Impact on the evolutions of geometric and electronic structures and chemical bondingcaused by different metals is presented. For the ground state of NbTa2O3-, three Oatoms are shown to prefer the bridging sites. The lowest energy structure of Nb2TaO3-has a terminal oxygen and two bridge oxygens. Ta3O3-possesses a perfect D3hsymmetry and multiple d-orbital aromaticity. But the mixed oxide clusters and Nb3O3-have low symmetry without aromaticity.